1. Field of the Invention
The present invention relates to an EL display device using organic EL elements and a method of manufacturing the same and, more particularly, to an EL display device having an integrated structure of organic EL elements and drivers for them, and a method of manufacturing the same.
2. Description of the Prior Art
Conventional organic EL elements will be briefly described with reference to FIGS. 1 and 2.
Metal electrodes 14 that form anodes, an organic thin film 13, and metal electrodes 12 that form cathodes are formed on a transparent substrate 11 to build organic EL elements. In this structure, positive and negative carriers are injected from the negative electrodes 12 and positive electrodes 14, respectively, to emit light toward the transparent electrodes.
As a prior art for integrating organic EL elements and drivers for them to form an EL display device, one is reported by C. C. Wu et al. of Princeton University in "Integration of Organic LED's and Amorphous Si TFT's onto Unbreakable Metal Foil Substrates", IEDM Tsch. Dig., 957-959, 1996 at International Electron Device Meeting (IEDM 196) held in December, 1996.
The structure of the EL display device reported in this meeting is as shown in FIG. 3. This EL display device is a single-layer device using a stainless steel substrate 15. A Pt electrode 16 and a thin Ag electrode 18 having a thickness of 150 .ANG. are formed as the anode and cathode, respectively, and a polyvinyl carbazole (PVK)-based polymer thin film 17 is formed as an organic film by spin coating. Emitted light is output to the thin Ag electrode 18 side.
In the structure shown in FIG. 3, although the organic EL element and the driver are integrated, the reported luminous efficiency of the organic EL element is as small as about 0.01%, which is by no means comparable to the luminous efficiency of 4% to 5% which is realized by a single organic EL element. Therefore, this EL display device was not evaluated except that it proposed an organic EL element on a new concept.
C. C. Wu et al. subsequently reported the structure of an EL display device shown in FIG. 4 at International Symposium (SID 197) of SID (Society for Information Display) held in May, 1997.
The structure shown in FIG. 4, regarding its stainless steel substrate 15, a Pt electrode (anode) 16, and a polymer thin film 17, is identical to that shown in FIG. 3 which is reported at IEDM '96. The difference of this structure resides in that the cathode is improved by fabricating it as a multilayer electrode consisting of a 150-.ANG. MgAg film and a 400-.ANG. ITO film, e.g., an ITO/thin MgAg laminated electrode (cathode) 19. According to the report, this improvement increased the luminous efficiency by about 1%.
In the structures shown in FIGS. 3 and 4, emitted light is output through the cathode. The structure shown in FIG. 3 uses an Ag film, which is in no way transparent, as a cathode. The thickness of this Ag film is decreased to obtain certain transparency. In the structure shown in FIG. 4, a transparent ITO film is used to provide an improvement in this point. However, the ITO film has an excessively large work function to make electron injection difficult (disadvantageous) due to ionization potential of the organic thin film. This difficulty is removed by interposing a thin MgAg film. In this case, although the emission efficiency is improved, since the MgAg film is by no means transparent, either, it decreases transmittance all the same.
A still another prior art is described in Japanese Unexamined Patent Publication No. 61-231584.
FIG. 5 is a perspective view showing the structure of an EL display device described in Japanese Unexamined Patent Publication No. 61-231584.
In the structure shown in FIG. 5, inorganic EL elements using an inorganic electroluminescent material, e.g., Zn:Mn, is formed on one major surface of a ceramic substrate 21. In order to connect the inorganic EL elements with drivers 27 integrally formed on the other surface of the ceramic substrate 21, interconnections that are connected to the inorganic EL elements on one side and to the drivers 27 on the other side are formed to extend through the ceramic substrate 21. This realizes integration of the inorganic EL elements and the drivers.
The structure shown in FIG. 5 is manufactured in the following manner.
As the inorganic EL elements of this EL display device, first electrodes 22 are formed on one major surface of the ceramic substrate 21, and subsequently an insulating layer 23, an inorganic emission layer 24, an insulating layer 25, and second electrodes 26 are sequentially formed.
Since emitted light is output to the second electrodes 26 side, transparent electrodes are used as the second electrodes 26. The material that forms the inorganic EL elements has high heat resistance. After the inorganic emission layer 24 is formed, the transparent second electrodes 26 are formed in accordance with ordinary sputtering.
Whereas the material that forms the inorganic EL elements has high heat resistance, the material of the organic EL elements lacks heat resistance. The difference in heat resistance of the material is one factor that has interfered with integration of the organic EL elements and their drivers.
Although the structure shown in FIG. 5 and the structure according to the present invention are similar in terms of integration, they are completely different in that this integration is enabled for organic EL elements in the present invention.
An inorganic EL element and an organic EL element have different emission mechanisms. Accordingly, whereas the inorganic EL element requires a drive voltage equal to or higher than 100 V, the organic EL element can be driven with a voltage of equal to or lower than 10 V. The reason the inorganic EL element requires a high drive voltage is that, unlike in the organic EL element, the inorganic EL element is not excited by recombination, but electrons collide against luminescent centers by electric field acceleration to emit light. The organic EL element has gained interest in terms of this drive voltage as well. However, the material of the organic EL element lacks heat resistance, as described above, and is conventionally difficult to integrate the organic EL element with the driver.
The temperature must be maintained to be equal to or lower than 80.degree. C. throughout the entire process of the manufacture of the organic EL element, which is a very large limitation. For this reason, although the various advantages of integration, e.g., down-sizing, weight reduction, and cost reduction, are sufficiently recognized, conventionally, the structure as an EL display device is barely realized by using a polymer having comparatively high heat resistance among the organic EL element materials and forming an ITO film in accordance with RF magnetron sputtering by limiting the condition, as in the examples shown in FIGS. 3 and 4. When a polymer is compared with a low molecule-based organic EL element material widely used as the organic EL element material, although its heat resistance is better, it is not suited for vacuum deposition that can achieve good film formation, and it can only be applied to spin coating and the like.
Attempts have been constantly made to form a transparent electrode at low temperatures. A typical prior art related to this is the technique described in Japanese Unexamined Patent Publication No. 9-71860.
This technique is mainly based on the need for formation of an ITO electrode on a plastic substrate at a low temperature, and provides an improvement over the sputtering target. A target manufactured by mixing indium oxide and zinc oxide in an oxide of an element having a valance equal to or larger than +3 as needed, molding and sintering the mixture, and annealing the sintered body, and a method of manufacturing the same are known. As a practical example of the manufacture of a target, for example, a case is reported wherein film formation is performed under the conditions shown in Table 1 by using a target manufactured by mixing 254 g of In.sub.2 O.sub.3 having a purity of 99.99% and an average particle diameter of 1 .mu.m, 40 g of zinc oxide powder having a purity of 99.99% and an average particle diameter of 1 .mu.m, and 6 g of titanium oxide powder having a purity of 99.99% and an average particle diameter of 1 .mu.m.
In Table 1, the substrate temperature is room temperature. This probably means, in the absence of specific description, that the substrate is not heated or cooled particularly. During sputtering, the temperature of the substrate naturally rises due to excessive energy of the film formation particles.
TABLE 1 Item Content, condition target size diameter: 4 inches thickness: 5 mm discharge method DC magnetron discharge current 0.3 A discharge voltage 450 V background pressure 5 .times. 10.sup.-4 Pa introduced gas Ar + O.sub.2 gas mixture pre-sputtering pressure 1.4 .times. 10.sup.-1 Pa pre-sputtering time 5 minutes sputtering pressure 1.4 .times. 10.sup.-1 Pa sputtering time 1.5 to 20 minutes substrate rotation speed 6 rpm substrate temperature room temperature